Luminescent materials with dual-mode excitation and tunable emission color for anti-counterfeiting applications

GdVO4-based dual-mode phosphors were successfully synthesized via a hydrothermal approach. The X-ray diffraction analysis determined the tetragonal structure as well as I41/amd space group of products by comparing with a reference pattern no. ICDD #01-072-0277. The morphology of yielded phosphors was confirmed by transmission electron microscopy and scanning electron microscopy. Detailed spectroscopy analysis revealed tunable luminescence properties with an increasing Yb3+ content in series of GdVO4: x% Yb3+, y% Tm3+, 5% Eu3+ (x = 5, 10, 15, 20; y = 0.1, 0.5, 1) phosphors. For Yb3+, Tm3+, and Eu3+- codoped phosphors we observed bands related to the 1G4 → 3H6 and 1G4 → 3F4 transitions of Tm3+ ions, occurred through the cooperative up-conversion mechanism, where two nearby Yb3+ ions were involved in near-infrared absorption. Moreover, the GdVO4: 20% Yb3+, 0.5% Tm3+, 5% Eu3+ showed the most outstanding color tunability from red color (x = 0.6338, y = 0.3172) under UV to blue color (x = 0.2640, y = 0.1988) under NIR excitation, which can be applied in anti-counterfeiting activity.


Scientific Reports
| (2023) 13:10773 | https://doi.org/10.1038/s41598-023-37608-w www.nature.com/scientificreports/ In this study, we aim to provide a comprehensive understanding of the properties and applications of the luminescent marks based on doped orthovanadates. All of above-mentioned features of the GdVO 4 : Yb 3+ , Tm 3+ , Eu 3+ phosphors synthesized by the feasible hydrothermal approach, designate as perfect for the anti-counterfeiting applications as the color tunability within the same material is difficult to falsify. The study introduces dual-mode orthovanadates as a viable substitute for fluoride-based materials in the realm of anti-counterfeiting tags. Through precise selection of dopant ions and host matrix, the resulting luminophores undergo up-conversion processes and demonstrate robust emission capabilities owing to charge transfer phenomena between O 2and Eu 3+ ions. These distinct mechanisms give rise to diverse color emissions and various wavelength excitation. Moreover, the orthovanadate materials exhibit remarkable resilience to harsh environmental conditions, including elevated temperatures, thereby rendering them advantageous by comparison to the aforementioned fluorides, which are more prone to decomposition.
A potential application of the implementation of a orthovanadates modifier into cellulose fibers for paper and fabric markings has been confirmed by the patent application submitted 43,44 . By showcasing the real-world application of our material, we establish its potential for practical implementation and highlight its relevance to various industries. Finally, according to our knowledge, this is the first time that Yb 3+ , Tm 3+ , Eu 3+ dopants are incorporated into the orthovanadate matrix, and its structural followed by spectroscopic properties are determined. It fills a gap in the existing literature by presenting an innovative approach that has practical implications and opens new avenues for future research and development.

Experimental section
Materials. Ammonium

Methods.
A series of GdVO 4 : x% Yb 3+ , y% Tm 3+ , 5% Eu 3+ (x = 5, 10, 15, 20; y = 0.1, 0.5, 1) was obtained in hydrothermal conditions. The concentration and the type of dopants was altered based on our knowledge and the literature in order to observe efficient emission processes [24][25][26][27][28] . In addition, the concentration of Yb 3+ , Tm 3+ and Eu 3+ dopant ions in prominent GdVO 4 host was altered to provide the intense, dual-mode luminescence under UV and NIR irradiation for anticounterfeiting applications. The composition of Ln 3+ ions used was selected to ensure the emission color dependent on excitation wavelength thus considered material is more difficult to replicate.
The synthesis was performed in Berghof autoclave (max. pressure 200 bar, additional stirring). All of the substrates were used as water solutions. The stoichiometric combination of 0.25 M Ln(CH 3 COO) 3 was mixed with the 0.1 M NH 4 VO 3 added dropwise under continuous stirring for 30 min. Resultant transparent mixture (pH 4.7) was then transferred to Teflon vessel and put for hydrothermal process under 180 °C for 3 h which yielded with yellow powder. Next, when the autoclave was naturally cooled to room temperature, the product was collected by centrifugation, washed with 1:1 mixture of deionized water and ethanol. Finally, the product was dried at 80ºC for 24 h for further analysis.
Characterization. The structural analysis was conducted with Bruker AXS D8 Advance powder X-ray diffractometer equipped with Johansson's monochromator and Lynx Eye strip detector, whereas the measurements were performed with Cu-K α 1 λ = 15,418 Å radiation within the 10-60 2Θ range, 0.05°/s step size. Morphology of studied materials was investigated with the use of transmission electron microscopy, TEM (JEOL 1400 with acceleration voltage of 80 kV) as well as scanning electron microscopy, SEM (Quanta 250 FEG, FEI equipped with EDAX detector). Luminescence properties were studied in terms of photoluminescence and upconversion luminescence, i.e. under UV or NIR excitation. The former was studied with the use of Hitachi F-7000 spectrofluorimeter, equipped with xenon lamp excitation source. The latter phenomenon was studied in terms of emission, luminescence decay and the number of photons involved in the process, with the use of PIXIS:256E Digital CCD Camera equipped with SP-2156 Imaging Spectrograph (Princeton Instruments), Mixed Domain Oscilloscope-200 MHz-Tektronix MDO3022 as well as the excitation source of CNI NIR 2W LASER 975 nm. All of the spectroscopic measurements were conducted at 293 K.

Results and discussion
Structure and morphology. Behind every specific luminescence feature, there are structural and morphological reasons as well. As seen in Fig. 1, the replacement of 25.5% of Gd 3+ ions with dopant ions in the host structure did not cause severe lattice distortions since the ionic radius of Eu 3+ is similar, while the Yb 3+ and Tm 3+ radii are smaller than the one of Gd 3+29 . The synthesized compounds are confirmed to be of GdVO 4 I4 1 /amd tetragonal zircon-type (ZrSiO 4 ) structure with the cell parameters of a = b = 7.2126 Å, c = 6.3483 Å, according to the reference pattern no. ICDD #01-072-0277 30,31 . The tetragonal and polyhedral structure of GdVO 4 are presented in Fig. 2c. Here, vanadium atom of [VO 4 ] 3− is tetrahedrally coordinated with O 2− ions, whereas the Ln 3+ are surrounded by eight oxygen atoms in a distorted dodecahedron structure 32 . The presence of sharp, narrow reflexes indicates discussed materials as highly crystalline and bulk. By the fact that none of the additional peaks are observed, synthesized powders are monophased and the replacement of Gd 3+ by Ln 3+ dopants was successful. What has to be said, an increased grain size growth as well as excellent crystallinity are caused by inevitable high temperature annealing (900 °C) which was applied to induce the UC luminescence, initially diminished by structure defects, typical for materials synthesized in hydrothermal conditions 33 . These features were further confirmed with the use of TEM and SEM methods (Fig. 2a,b) 11,37,38 . In this research, the intensity of hypersensitive 5 D 0 → 7 F 2 is the highest among the Eu 3+ emission bands which indicated the low symmetry around Eu 3+ ions 38 . What is more, with an increasing Yb 3+ concentration, the intensity of both excitation and   Upconversion luminescence. It is essential for dual-mode luminescence (ergo inimitable anti-counterfeiting materials) to be intense both under UV and NIR irradiation. According to the upconversion spectrum depicted in Fig. 4, the energy transfer between Eu 3+ and Tm 3+ ions is observed as variety of Eu 3+ and Tm 3+ emission bands are present in the spectrum. What has to be noted, the population of Eu 3+ via inefficient phononassisted-Yb 3+ -Eu 3+ is barely observed; here, Tm 3+ acts as an energy mediator between the sensitizer (Yb 3+ ) and emitter (Eu 3+ ) 22 . According to that, several emission bands associated to the Tm 3+ and Eu 3+ are observed in the spectrum, which vary with intensity, namely 1 G 4 → 3 H 6 (Tm 3+ , ~ 478 nm), 5 D 1 → 7 F 1 (Eu 3+ , ~ 521 nm), 5 D 1 → 7 F 2 (Eu 3+ , ~ 552 nm), 5 D 0 → 7 F 1 (Eu 3+ , ~ 590 nm), 5 D 0 → 7 F 2 (Eu 3+ , ~ 615 nm), 1 G 4 → 3 F 4 (Tm 3+ , ~ 650 nm), 5 D 0 → 7 F 4 (Eu 3+ , ~ 700 nm) as well as the most intense 3 H 4 → 3 H 6 (Tm 3+ , ~ 800 nm) band. The last-mentioned transition is observed in NIR region of spectrum thus it does not influence the emission color. What has to be mentioned,   Table 2. With an increasing Yb 3+ content, the luminescence color shifts from the red towards purple and blue region of CIE chromaticity diagram. By the gradual substitution of Gd 3+ by Yb 3+ , the distance between sensitizer and emitters such as Tm 3+ , Eu 3+ decrease. As mentioned before, the efficiency of Yb 3+ -Eu 3+ transfer is low thus the Yb 3+ -Tm 3+ transfer is favored here. In this case, the competitive Tm 3+ -Eu 3+ absorption is decreased, which is also connected with the lower intensity of Eu 3+ emission bands in the UC spectrum, as well as the red component of the luminescence color 23 . Also, when Yb 3+ -Tm 3+ is greatly enhanced, the relative intensity between blue and red emissions of Tm 3+ is increased, which results with the outcoming blue upconversion luminescence 40,41 . The population of Eu 3+ and Tm 3+ excited levels, as well as the energy transfer between these species were studied in terms of luminescence decay. Here, due to the color change, the most important bands are the ones in the blue and red region of spectrum. Based on that, the greatest attention was put to the lifetime of 1 G 4 → 3 H 6 (Tm 3+ , ~ 478 nm), 5 D 0 → 7 F 2 (Eu 3+ , ~ 615 nm) and 1 G 4 → 3 F 4 (Tm 3+ , ~ 650 nm). As shown in Table 3, the luminescence lifetime of Eu 3+ is decreasing with an increasing Yb 3+ content. Based on that, the possibility of Tm 3+ -Eu 3+ is diminished, whereas Yb 3+ -Tm 3+ transfer is favored. It is also confirmed by the enhanced blue emission of Tm 3+ since less energy is transferred towards Eu 3+ site of lattice. Moreover, in order to derive the average number of   www.nature.com/scientificreports/ photons (n) involved in the upconversion process, a laser power dependent luminescence study was performed (Fig. 6). Interestingly, the slopes (n) of 1 G 4 → 3 H 6 and 1 G 4 → 3 F 4 transitions suggest the involvement of two photons regardless the Yb 3+ concentration. In the contrary to the common understanding of the 1 G 4 population mechanism, i.e. through threephoton-absorption, meant also as sequential sensitization, in the case of GdVO 4 : x% Yb 3+ , 0.5% Tm 3+ , 5% Eu 3+ a cooperative sensitization is in fact happening. In this mechanism there are two Yb 3+ ions involved, which absorbs photons in order to promote themselves towards 2 F 5/2 excited state. Then, a coupled cluster state of two Yb 3+ ions formed transfers energy towards Tm 3+ which results with the population of its 1 G 4 level 42 .
To summarize all studied processes, a mechanism of upconversion in GdVO 4 : x% Yb 3+ , 0.5% Tm 3+ , 5% Eu 3+ systems can be proposed in Fig. 7. The whole phenomenon begins under 975 nm CW excitation, when the energy is first absorbed by two nearby Yb 3+ ions. This results with the promotion of sensitizers from their 2 F 7/2 -2 F 5/2 level. At this point, two simultaneous processes are happening. Due to the formation of coupled Yb 3+ cluster state, another photon is absorbed and transferred towards 1 G 4 , from which 478 nm and 650 nm emissions occur. Also, there is a photon transferred from this level towards Eu 3+ 5 D 1 , where after energy dissipation to 5 D 0 , several Based on its pristine tunable luminescence properties, sample composed of GdVO 4 : 20% Yb 3+ , 0.5% Tm 3+ , 5% Eu 3+ was chosen for fibers preparation. Then luminescent fibers were used for paper modification and fabric production as an example of anti-counterfeiting application 43,44 . Regardless the medium, the outcoming luminescence color as well as its intensity remain unchanged. These properties recommend GdVO 4 : 20% Yb 3+ , 0.5% Tm 3+ , 5% Eu 3+ phosphor for anticounterfeiting applications performed in patent proposal submission. In Fig. 8, an actual luminescence color under different excitation sources is present.

Conclusions
To conclude, the pre-eminent, spectroscopic properties of GdVO 4 : 20% Yb 3+ , 0.5% Tm 3+ , 5% Eu 3+ define this material as the excellent one for anti-counterfeiting purposes. In our study, we investigated different Yb 3+ concentration and its influence on structural and spectroscopic properties. With an increased sensitizer content, the upconversion luminescence is more intense whereas its color is tuned towards blue region of spectrum. What is more, in Yb 3+ /Tm 3+ /Eu 3+ system, 1 G 4 → 3 H 6 and 1 G 4 → 3 F 4 emissions result from twophoton excitation in terms of cooperative sensitization where two nearby Yb 3+ ions are involved in IR excitation absorption process. Also, varied luminescence color under different sources of excitation qualified GdVO 4 : 20% Yb 3+ , 0.5% Tm 3+ , 5% Eu 3+  www.nature.com/scientificreports/ phosphor for anticounterfeiting application. In applied cellulose medium, luminescence color and intensity remained unchanged in comparison to the phosphor in the powder state.

Data availability
The datasets generated during and analyzed during the current study are available from the corresponding author on reasonable request.